4-16
888-2595-001
12/20/11
WARNING: Disconnect primary power prior to servicing.
Section 4 Theory of Operation
ZX Series
8.
PWR_CTL line to exciter or IPA bias control (U28C)
9.
RF amplification by IPA-PA modules with power set by exciter or IPA bias.
10.
Transmitter output (loop complete).
Comparators U25C and U25D compare the forward and reverse power samples
entering the APC circuit against a fixed +2.25V reference from voltage divider R107-
R35 and signals a fault when the forward sample is too low (low gain) or the reverse
sample is too high (reverse foldback).
Potentiometer R13 adjusts the gain applied to the reverse power sample by variable
gain amplifier U24A, thereby setting the threshold where the reverse power foldback is
invoked by the APC circuit.
4.12.3 Operating Mode and Bias Level Control
The transmitter is capable of operating in any one of three modes based on incoming
command lines from the exciter. The transmitter determines its operating state via the
FM_ON_STATUS and HD_ON_STATUS lines available at the transmitter interface
connector of the Harris FlexStar HDX exciter. These lines are pulled low by the exciter
when the FM carrier or HD carriers are present at the exciter output. These lines are
passed via the I/O flter board to the controller via the FM_ON_BUS and HD_ON_BUS
signal lines.
The grounding of the FM_ON_BUS status line causes the gate of transistor Q2 to go
low, thereby applying a logic high signal to input A of 4-bit decoder IC U12. A similar
operation takes place with the HD_ON_BUS line, Q3, and U12-B input. The status of
these two lines can select on of three possible transmitter operation modes: FM,
FM+HD, or HD/SL according to the following truth table:
The FlexStar exciter split level mode is mapped to the transmitter HD mode via steering
diode CR8. This was done because a transmitter operating in split level mode (FM+HD
Table 4-1 Operating Mode Table
FM_ON_BUS
HD_ON_BUS
FlexStar Mode
TX Mode
false (high)
false (high)
Split Level Mode
HD/SL Mode
true (low)
false (high)
FM Mode
FM Mode
false (high)
true (low)
HD Mode
HD/SL Mode
true (low)
true (low)
FM+HD Mode
FM+HD Mode
Содержание ZX10
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Страница 80: ...3 12 888 2595 001 12 20 11 WARNING Disconnect primary power prior to servicing Section 3 Operation ZX Series ...
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